The invention relates to a thread monitoring mechanism.
Thread monitoring mechanisms are arranged in the thread run of a textile machine, and, in general can be differentiated into non-contact monitoring mechanisms and those, as in the case with which the invention is concerned, which butt in sensing manner against the thread, and are called thread sensors.
Conventional thread sensors emit a Yes/No signal, namely "Thread is present" or "Thread is not present". Upon the production of twisted yarn, at least two thread components participate in the total thread, so that, besides the knowledge regarding the Yes/No state, also knowledge regarding the presence of only one thread component is desirable. The same also holds true for so-called cabling threads, which contain two yarn components which run in over different yarn paths and which combine only underneath the balloon thread guide in the so-called cording triangle. In both cases it could, upon breakage of only one yarn or respectively thread component, lead to a condition in which the second yarn component holds the thread sensor in its operating position, so that the thread sensor does not respond and thus only one yarn strand is wound.
In order to prevent such a faulty winding, so-called differential thread tension sensors are provided, i.e. thread sensors which not only have the Yes/No function as their content, but also react upon a variation of the overall tension, for example upon omission of one yarn component.
A typical differential thread tension sensor for a cabling machine is described for example in DE-OS 29 39 435. This known thread sensor generally comprises a swivel lever which is mounted on a swivel axis and on which a thread guidance roller, overrun by the thread that is to be monitored, is mounted. The swivel lever is supported in the operating position corresponding to an orderly thread run by a helical compression spring which can be adjusted by means of an adjusting screw in such a way that also upon breakage of one thread or respectively yarn component, and thus e.g. halved thread tension, the swivel lever is swivelled, under the action of the helical compression spring, into a switching position which interrupts the operation of the thread processing location, although the other yarn component continues to run over the sensing roller, i.e. upon tearing of a single one of the two yarn or respectively thread components the thread processing location is stopped. It is self-evident that upon tearing of both yarn components likewise a shut-down of the thread processing location is effected.
Described in DE-AS 15 35 167 is a thread sensor in the form of a two-armed swivel lever on the one arm of which a tension spring acts, whilst the other arm carries a sensing pin which is overrun by the thread. This thread sensor is designed as a differential thread tension sensor, which however also reacts upon overload, i.e. this thread sensor responds both upon thread breakage or falling-below of a predetermined thread tension on the one hand and upon the exceeding of a predetermined thread tension on the other hand, in that for example upon abatement of the thread tension the swivel lever is swivelled, under the effect of the spring acting on it, in the clockwise direction, whilst upon an increase in the thread tension the swivel lever is swivelled contrary to the force of the tension spring in the anticlockwise direction.
A disadvantage of these two known spring-loaded thread sensors is that an adaptation or variation of the spring force aimed at a specific thread tension is possible only in the region of an individual thread sensor, which, more particularly in the case of multi-location machines is complex and time-consuming.